The efficiency of radio and microwave power amplifiers (PA) constantly needs to be improved to reduce their power consumption. The telecom industry energy consumption is estimated to account for approximately 1 percent of the yearly global energy consumption of the planet, and a large share of this energy consumption is assigned to radio base stations (RBS). It is therefore important with high efficiency Power Amplifiers. Within the RBS, more than 80% of the energy consumption is related to radio equipment, amplifiers and cooling systems, a figure that can be reduced by increased power amplifier efficiency.
In modern communication standards, such as 3G, 4G, LTE, WiMax etc, the desired output signal has a high peak to average ratio (PAR) of typically 7-10 dB. To obtain high average efficiency for Power Amplifiers operating in such systems, the efficiency of the PA must remain high also below the peak output power. A variety of amplifier and transmitter topologies have been proposed to address this problem. One of the most popular and high performing topologies is the Doherty amplifier.
Furthermore, the increasing number of frequency bands and spectrum fragmentation in modern communication systems require Power Amplifiers with broadband capabilities. However, it has turned out to be very difficult to design Power Amplifiers with both broadband capabilities and high average efficiency at the same time.
When realizing a Doherty amplifier, there are today two main techniques that are used.
The first technique is to drive the amplifier with a single RF-input, as originally proposed by Doherty. To obtain the correct load modulation the main amplifier is typically biased in class AB or class B while the auxiliary amplifier is biased in class C. This gives a simple topology but the linearity and efficiency as well as the power utilization factor (PUF) of the amplifier are deteriorated compared to the theoretical limit of the Doherty amplifier. Class A, B, AB and C amplifiers are well known to the skilled person and therefore not further discussed here.
The second technique is to use dual RF-inputs, where each RF-signal is modulated independently. This allows the transistors in the amplifiers to be biased in class B and thereby, in theory, the possibility to obtain ideal Doherty characteristics. Although this technique requires more complex signal processing and two RF input signal paths it is popular due to the good Doherty characteristics achievable.
The topology of an ideal Doherty amplifier is schematically depicted in FIG. 1, where ideal current sources represent main and auxiliary amplifiers. Throughout the description and claims, the current values given refer to the amplitude of the fundamental frequency, if nothing else is stated. The fundamental frequency is defined as the lowest frequency of a periodic waveform. Also, all harmonic frequencies are assumed to be short circuited throughout the description and claims. The values of a load impedance, ZL, a main amplifier transmission line characteristic impedance, Zc, a drain-source bias, Vds, and amplifier fundamental frequency RF output current amplitudes, Ia for the auxiliary amplifier and Im, for the main amplifier for proper operation are given in Table 1 below.
TABLE 1Basic ideal Doherty amplifier load impedance, transmission linecharacteristic impedance, drain-source bias and amplifier output currents.Vds,a = Vds,m(1)ZL = xbVds,a/Imax,m(2)Zc = Vds,a/Imax,m(3)Im = ImImax,m 0 < Im < 1(4)       I    a    =      {                                        0            ,                                                0            ⁢                                                  ≤                                                  ⁢                                          I                m                            _                        ≤                          x              b                                                                                                                        kI                                      max                    ,                    m                                                  ⁡                                  (                                                                                    I                        m                                            _                                        -                                          x                      b                                                        )                                            ⁢                              ⅇ                                  i                  ⁢                                                                          ⁢                  θ                                                      ,                                                              x              b                        <                                          I                m                            _                        ≤            1                              (5) θ = π f/2(6)   k  =      1          x      b      (7)Imax,m is the maximum main amplifier output current amplitude at the fundamental frequency, Vds is the drain-source bias and Im and Ia is the normalized main and auxiliary amplifier output current amplitude at the fundamental frequency, respectively. Added subscript m or a, refers to the main and auxiliary amplifiers, respectively.
Im is henceforth, unless otherwise stated, called the main amplifier output current and Ia is henceforth, unless otherwise stated, called the auxiliary amplifier output current. θ is the phase difference, expressed in radians, between the auxiliary amplifier output current Ia and the main amplifier output current Im. Table 1 is adapted for transistors of FET-type (Field Effect Transistors) having gate, drain and source terminals. Designations and terms are thus adapted for FET transistors. These terms and designations are however also valid for transistors using base, collector and emitter terminals, i.e. with base corresponding to gate, collector to drain and emitter to source. Some examples; whenever drain-source is mentioned it is also valid for collector-emitter and whenever drain-source bias or drain-source voltage is used it is also valid for collector-emitter bias or collector-emitter voltage and when gate biasing is mentioned it is also valid for base biasing. All examples mentioned are thus also valid for corresponding examples using transistors with base, emitter and collector terminals.
For a given technology certain design rules apply. In this case the values of the drain-source bias Vds,i, the load impedance ZL, and the main amplifier transmission line characteristic impedance Zc must be chosen to fulfill Ipeak,i≦Isat,i and Vpeak,i≦Vb,i, where Ipeak,i is the amplifier peak output current, Isat,i is the amplifier saturation output current, Vpeak,i is the maximum amplifier output voltage and Vb,i is the amplifier break down voltage. Note that Ipeak,i, Isat,i, Vpeak,i and Vb,i all refer to time domain peak values including fundamental and harmonic frequencies as well as possible DC current. The subscript i refers to the main or auxiliary amplifier. These design rules are well known for the design of amplifiers and thus well known to the skilled person. As the subscript i refers to the main or auxiliary amplifier, it means that the current and voltage restrictions are valid for both the main amplifier and the auxiliary amplifier. Imax,m is the maximum main amplifier output current amplitude at the fundamental frequency. Both amplifiers are arranged to operate over a bandwidth B with a centre frequency f0 where f is the normalized frequency, defined as the frequency divided with the centre frequency f0. The parameters xb and k will be described in association with FIG. 2. The definitions of the ideal Doherty amplifier according to table 1 is well known to the skilled person and can e.g. be found in “RF Power Amplifiers for Wireless Communication” by Cripps, Steve C, second edition, Chapter 10. Henceforth an ideal Doherty amplifier is called a Doherty amplifier unless otherwise stated.
FIG. 1 shows a power amplifier 100 comprising the main amplifier 101 and the auxiliary amplifier 102, each amplifier, in FIG. 1 represented with an ideal current source connected to a ground 104 at a first end and delivering an output current at a second end. The amplifiers are arranged to receive an input signal each and to operate in parallel and at an output end 105 both being connected to one end of a load having an impedance ZL the other end of which is connected to the ground 104. Both amplifiers are, as mentioned above, arranged to operate over a bandwidth B with a centre frequency f0. The main amplifier 101 is arranged to generate the main amplifier output current Im and is connected to the load with the impedance ZL via a main amplifier transmission line 103 with the characteristic impedance Zc. The load with the impedance ZL is henceforth, unless otherwise stated, called the load ZL. The main amplifier transmission line has an electrical length of a quarter of a wavelength at the centre frequency f0 and has an output impedance Zm2 towards the load ZL and an input impedance of Zm1 to the main amplifier transmission line 103. The auxiliary amplifier 102 is arranged to generate the auxiliary amplifier output current Ia and is connected to the load ZL as described. The main amplifier is arranged to operate with a drain-source break down voltage Vb,m and with a drain-source bias Vds,m. The auxiliary amplifier is arranged to operate with a drain-source break down voltage Vb,a and with a drain-source bias Vds,a. The power amplifier is arranged to deliver an output power Pout, and the auxiliary amplifier output current Ia is arranged to be set to zero below a First Efficiency Peak, FEP. The FEP is defined as the first efficiency peak in the power ratio Pout to Pout,max. Pout,max is the maximum power output of the power amplifier and PFEP defines the power ratio at the First Efficiency Peak. All harmonic frequencies are assumed to be short circuited.
FIG. 2 shows how the efficiency depends on output power at the centre frequency f0 for a class B biased Doherty amplifier. The efficiency is defined as a ratio between the output power Pout of the power amplifier and the supplied DC power to the power amplifier. The ratio Pout/Pout,max, where Pout,max is the maximum output power from the power amplifier, at the first efficiency peak (FEP), referred to as PFEP (see FIG. 2) and often expressed in dB below Pout,max, is a design parameter that sets a condition on the load impedance ZL as given by equation (2) where xb=10PFEP/20. xb can be seen as a transformation of a logarithmic PFEP value to a current amplitude being defined as a normalized main amplifier fundamental frequency output current amplitude Im, henceforth called the normalized main amplifier output current, at PFEP. The normalized main amplifier output current Im is defined as Im in relation to a maximum current through the main amplifier, Imax,m, see equation (4) where Imax,m is the maximum main amplifier output current amplitude at the fundamental frequency. k is the inverse of xb. The ratio Pout/Pout,max is defined as the normalized output power of the power amplifier. The normalized auxiliary amplifier output current Ia is defined as Ia in relation to the maximum current through the main amplifier, Imax,m.
In FIG. 2 the efficiency in percentage, depicted on the y-axis 202, is illustrated with a graph 204 as a function of the ratio Pout/Pout,max, shown on the x-axis 201 in dB below Pout,max. PFEP is shown as the FEP, 205 in the graph 204. How to choose PFEP depends on the signal probability density function, and typically, PFEP should approximately be equal to or slightly below the signal Peak to Average (PAR) value.
The transmission line on the output of the main amplifier transforms Zm2 to Zm1 according to equation (8):
                                          Z                          m              ⁢                                                          ⁢              1                                ⁡                      (                          f              _                        )                          =                              Z            c                    ⁢                                                    Z                                  m                  ⁢                                                                          ⁢                  2                                            +                              j                ⁢                                                                  ⁢                                  Z                  c                                ⁢                                  tan                  (                                                                                    f                        _                                            ⁢                                                                                          ⁢                      π                                        2                                    )                                                                                    Z                c                            +                              j                ⁢                                                                  ⁢                                  Z                                      m                    ⁢                                                                                  ⁢                    2                                                  ⁢                                  tan                  (                                                                                    f                        _                                            ⁢                                                                                          ⁢                      π                                        2                                    )                                                                                        (        8        )            where f is the normalized frequency, defined as the frequency divided with the centre frequency f0. This formula for impedance transformation is well known to the skilled person and therefore not further discussed here.
Clearly Zm1 is frequency dependent unless Zm2 is equal to Zc, which only is the case at Pout,max. This frequency dependency is a major reason to the often reported small bandwidth of the Doherty amplifier, i.e. when component values, currents and voltages are dimensioned according to Table 1. The bandwidth limitation of the Doherty amplifier has been examined and it has been shown that the efficiency has a strong frequency dependency at PFEP while the efficiency is independent of frequency at Pout,max. This is illustrated in FIG. 3 showing efficiency in percentage on the y-axis 302 versus output power as the ratio Pout/Pout,max on the x-axis 301 in dB below Pout,max for a Doherty amplifier with xb=0.5. The Doherty amplifier is dimensioned with values calculated according to Table 1 and is class B biased, for both the main and auxiliary amplifier with optimal performance at the center frequency f0. The efficiency versus output power for a typical standard amplifier dimensioned and biased as described above and with Doherty topology is plotted as three graphs 303-305 for a range of frequencies. The first graph 303 represents the centre frequency f0, the second graph 304 frequencies at 0.8 f0 and 1.2 f0 and the third graph 305 frequencies 0.6 f0 and 1.4 f0. The Pout,max value is marked at a point 306 in FIG. 3. The Output Power Back-Off (OPBO) is the amount of back-off from this maximum power output. The OPBO is a consequence of the required amplitude modulation of the transmitted signal. OPBO is defined as the ratio Pout to Pout,max and is often expressed in decibels (dB) below Pout,max. The strong frequency dependency in power back off is evident from the graphs in FIG. 3 and as frequency deviates from f0 the efficiency curve degrades to regular class B amplifier efficiency behavior which is substantially below the optimal performance of a class B biased Doherty amplifier, as illustrated with the second graph 304 and the third graph 305 showing substantially less efficiency than the first graph 303.
It is important to note the xb dependency in equation (2) which implies that, in order to reconfigure PFEP, ZL, must be adjustable. Although this is highly desirable, it will require a tuneable matching network using e.g. varactor technology. In most practical implementations, this therefore makes PFEP a fixed parameter which limits the potential of using the Doherty amplifier in reconfigurable multi-standard applications.
The deterioration of efficiency in OPBO at frequencies outside the design frequency, typically being the centre frequency f0, and the lack of reconfigurable PFEP are both great drawbacks of the Doherty amplifier, dimensioned according to Table 1, in times when wideband/multi standard devices are highly requested.
There is thus a need for an improved power amplifier with both broadband capabilities and high average efficiency at the same time, preferably also with a possibility to conveniently reconfigure PFEP.